fix typo in v12 test isolation migration guide (#5156)

Author: markrmullan

docs/guides/references/migration-guide.mdx

@@ -174,7 +174,7 @@ test and verify it can run successfully without the test before it.
 
 #### Simulating Pre-Test Isolation Behavior
 
-Test isolation did no truly exist pre-12. Pre-Cypress 12, the behavior was a
+Test isolation did not truly exist pre-12. Pre-Cypress 12, the behavior was a
 hybrid of both `testIsolation` enabled and disabled. All local storage and
 cookies on the current domain were cleared, but Cypress did not clear session
 storage and the page always persisted.